Goal: The goal of this unique core will be to provide cost-effective high speed flow cytometry and related services to the Program Project members. Two major uses are anticipated: 1) rapid purification of high numbers of cells transfected with a fluorescent protein, typically for experiments involving multiple samples. The purified cells need to be >95% pure and will be used for biochemical, immunological, or short term cell culture assays. 2) rapid purification of murine hematopoietic stem cells and progenitor cells for subsequent infection with retroviruses containing an oncogene or signaling molecule. These cells are typically present in low numbers, and final purity needs to be 95-99%. They will be used either for in vitro cell culture assays (such as colony assays) or for re-injection in mice as part of a bone marrow transplant experiment. Sterile sorting is essential for both types of experiments.